Some stereo loudspeaker systems have right and left channels driving four speakers, wherein the four speakers include a pair of speakers on the right and another pair on the left. Each pair comprises a main speaker and an associated sub-speaker, so the system includes a right main speaker, a right sub-speaker, a left main speaker and a left sub-speaker. Examples of such systems are disclosed in U.S. Pat. Nos. 4,489,432 and 4,638,505, both of which are specifically incorporated herein by reference.
In some cases, the right and left channels feed R and L signals to the right and left main speakers, respectively. In addition, at lower frequencies, R and L signals are applied respectively to the right and left sub-speakers as well. At certain higher frequencies, however, a differential R-L signal is sent to the right sub-speaker, and a differential L-R signal is sent to the left sub-speaker. Although the proposed benefits of this are well known to those of ordinary skill in the art, actually achieving the desired results has been an elusive goal due to an assortment of problems pertaining to some frequencies, various signal conditions and/or certain amplifiers.
Various attempts to avoid some problems seem to create others. The '505 patent, for instance, discloses using a combination of capacitors and inductors for providing a desired response at both high and low frequencies; however, resonance and distortion seem to occur at a transitional point between high and low frequencies. In what perhaps is an attempt to avoid resonance created by a combination of capacitors and inductors, some known systems omit the capacitors of the '505 patent, add a crossover wire between the two sub-speakers, and rearrange the circuit such that two inductors, without the capacitors of the '505 patent, provide desired responses at both high and low frequencies. A prior art Polk Audio electrical schematic SDA 2B/CRS+ Schematic NC refers to such a crossover wire as an “IC wire from the crossover.” A crossover wire connecting two sub-speakers, however, can provide a low DC impedance connection between the right and left minus input terminals of the amplifier, which can create problems with certain amplifiers, particularly amplifiers of the bridge type output.
Other known prior art systems insert a capacitor along the shunt to break the shunt's otherwise low DC impedance path. Adding the capacitor, unfortunately, reintroduces the resonance issue as an undesirable artifact. Over certain frequencies (e.g., 50-200 Hz) and/or under certain conditions (e.g., when L=−R, or when L exists while R=0), the capacitor resonates with the series inductors, resulting in sudden shifts in phase and level changes in the sub-speakers. In some cases, the resonance of the added capacitor with the series inductors produces poor transient response in the 50-200 Hz range. The sudden shift in phase, level and poor transient response produce undesirable audible effects, such as objectionable coloration in the upper bass and lower midrange frequencies.